Enhancement of the photovoltaic performance of CH₃NH ₃PbI₃ perovskite solar cells through a dichlorobenzene-functionalized hole-transporting material

A dichlorobenzene-functionalized hole-transporting material (HTM) is developed for a CH₃NH₃PbI₃-based perovskite solar cell. Notwithstanding the similarity of the frontier molecular orbital energy levels, optical properties, and hole mobility between the functionalized HTM [a polymer composed of 2′-butyloctyl-4,6-dibromo-3-fluorothieno[3,4-b] thiophene-2-carboxylate (TT-BO), 3′,4′-dichlorobenzyl-4,6-dibromo-3- fluorothieno[3,4-b]thiophene-2-carboxylate (TT-DCB), and 2,6-bis(trimethyltin)- 4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b′]dithiophene (BDT-EH), denoted PTB-DCB21] and the nonfunctionalized polymer [a polymer composed of thieno[3,4-b]thiophene (TT) and benzo[1,2-b:4,5-b′]dithiophene (BDT), denoted PTB-BO], a higher power conversion efficiency for PTB-DCB21 (8.7-%) than that for PTB-BO (7.4-%) is achieved because of a higher photocurrent and voltage. The high efficiency is even obtained without including additives, such as lithium bis(trifluoromethanesulfonyl)imide and/or 4-tert-butylpyridine, that are commonly used to improve the conductivity of the HTM. Transient photocurrent-voltage studies show that the PTB-DCB21-based device exhibits faster electron transport and slower charge recombination; this might be related to better interfacial contact through intermolecular chemical interactions between the perovskite and the 3,4-dichlorobenzyl group in PTB-DCB21. Made to interact: A functionalized hole-transporting material (HTM) for improved interactions between a halide perovskite and HTM is reported (see picture). A dichlorobenzene-functionalized HTM improves the power conversion efficiency of a CH₃NH₃PbI₃ perovskite solar cell by 18.5-%.